Process for the preparation of printing plates and material for use therein



United States Patent V PROCESS FOR THE PREPARATION OF PRINTING Electrophotographic copies are prepared by electrostatically charging an electrophotographic copying material, exposing it to light under a master or episcopically, developing the resulting electrostatic image by dusting it over with a resin powder and subsequently fixing the image, e.g. by heating, to cause the resin powder to become firmly anchored to the support for the light-sensitive coating.

Printing plates may be prepared from such electrophotographic copies by treatment with a solvent which removes the coating from the support in the image-free areas, the bared parts being rendered hydrophilic, when necessary, and the image parts being inked up with greasy ink. The known process has the disadvantage that the image-bearing parts of the light-sensitive coating are partially dissolved away by the solvent, so that the resulting printing plates are of inferior quality.

Lithographic printing plates have also been prepared by applying to the support for the light-sensitive coating, a layer of a hydrophilic substance and, after producing, developing and fixing the electrophotographic image, dissolving away the photoconductive coating only. This has the disadvantage that a hydrophilic layer must be applied to the support before the photoconductive coating. Furthermore, when printing plates are being prepared, the hydrophilic layer must be bared in the image-free parts. This layer, however, tends, when the photoconductive coating is being removed, to retain fine particles of the coating so that the prints are often not free from background, but exhibit traces of ink in the background.

It has been proposed to render the image-free parts of an electrophotographic copy hydrophilic by treatment with solutions of Water-soluble salts of divalent metals of Groups I, II, IV, VII and VIII of the Periodic System, e.g. zinc acetate in an aqueous/alcoholic solution and a mixture of zirconyl acetate and sodium acetate in butanol, formaldehyde and water. This treatment does not, however, yield prints free from background.

Solutions of ferroand ferricyanides, to which acetic acid, tartaric acid or citric acid has been added, have also been suggested for this treatment. Printing plates so prepared have, however, the disadvantage that they tend to scum after producing only a few prints and generally yield only short printing runs.

The present invention provides a process for the preparation of a printing plate from a developed and fixed image produced electrophotographically upon electrophotographic material containing an inorganic photoconductor in its light-sensitive layer, which comprises treating the image, to render the non-printing areas hydrophilic, with an aqueous solution containing a boric acid and/or a salt of a boric acid, a hydrophilic organic acid of high molecular weight and one or more of the following additives: moisture-conserving agents, wetting and swelling agents, and hydrophilic film-forming substances. The solution may also contain a small proportion of a water-miscible organic solvent.

The process according to the invention has the advantage that it results in printing images which are completely free of background and which are capable of considerably longer printing runs than those hitherto obtainable. The image areas of the copy, which are covered with resin powder, remain hydrophobic after treatment with the aqueous solution and can accept greasy ink. Hydrophilic properties are imparted to the non-image areas of the copy very easily and quickly so that the treated electrophotographic copy can be set up directly in an offset printing machine, wiped over with the aqueous solution, and is then ready for printing.

However, printing can be deferred for some time after the copy has been treated with the solution as described above, and in this case no further wetting treatment is required before printing.

The following are exemplary of inorganic photoconductors which may be used: photoconductive oxides, sulfides, selenides, tellurides and iodides of zinc, bismuth, molybdenum, lead, antimony and cadmium. Zinc oxide is preferred. Mixtures of these photoconductors can also be used. A small proportion of an organic photoconductor can be utilized in conjunction with the inorganic photoconductors. Exemplary of such organic photoconductors are oxadiazoles, oxazoles, triazoles, and polymers such as poly-N-vinyl carbazole or poly-C-vinyl carbazole.

The light-sensitive coating may also contain a resinous binder. The resins used may be natural or synthetic, and the following are exemplary: polyvinyl chloride, polyvinyl acetate, polyvinyl alcohols, polystyrene, polycondensates such as phthalate resins, alkyd resins, maleinate resins, silicone resins, ketone resins, cournarone resins, indene resins, phenol resins modified with colophony, and processed natural substances such as cellulose esters and ethers. The light-sensitive coating may contain other additives such as activators, sensitizers, fillers and pigments.

The support for the light-sensitive coating may be paper or a foil of metal such as aluminum, zinc, copper or brass, or a foil of paper or synthetic plastic material with a vacuum-deposited or laminated metal surface. The paper may have a conductive precoat which is resistant to organic solvents, as described for example in US. Patents Nos. 2,534,650, 2,681,617 or 2,559,610.

In performing the process, the electrocopying material is charged by means of a corona discharge and is then exposed to light under a master. The material can also be exposed by episcopic projectionor it can be exposed in a camera. The resulting latent electrostatic image is developed with one of the usual pigmented resin powders, which may if desired be suspended in a dielectric liquid, and the developed image is fixed, e.g. by heating with an infra-red radiator to about -170 C., preferably 160 C., or by treatment with solvent vapors. In this way, a smear-proof image is obtained.

To convert the fixed image into a printing plate, it is treated with the aqueous solution described above. This may be effected by wiping, e.g. with a saturated cotton pad, by immersion in the solution, or the foil may be set up in an offset printing machine and wiped over with a sponge wetted with the solution. It is sometimes advantageous to wipe the image over several times with the solution with some pressure or rubbing action.

The boric acids which may be included in the solution are orthoboric acid, H 30 or metaboric acid, HBO Polyboric acids, in particular tetraand pentaboric acids, which in general can be prepared only in the form of their salts, can also be used. In aqueous solution these acids yield orthoboric acid, which is also obtained when boric acid anhydride B 0 is dissolved in water. The salts of boric acid which may be used include water-soluble salts of alkali metals, such as sodium and potassium, and alkaline earth metals, such as calcium and magnesium. The best results are obtained with ammonium as the cation. Alkali metal salts and the ammonium salts of tetraboric acid can be used, and these are present, as such, in the solution. Salts of metaboric acid, e.g. sodium metaborate,

and perborates, such as sodium perborate, can also be used. Mixtures of difierent boric acids, of different salts of boric acid, and of a boric acid and a water-soluble salt of a boric acid can also be used.

The preferred hydrophilic organic acid of high molecu lar weight is polyvinyl phosphonic acid, which may be partially esterified. Polymerization products of substituted and unsubstituted acrylic acids, such as polyacrylic acid or polymethacrylic acid can also be used.

Exemplary of moisture-conserving additives are'polybasic alcohols such as glycol, polyglycols, glycerine, polyv V polyvinyl alcohol and polyvinyl pyrrolidone.

Exemplary of water-miscible organic solvents which 7 can be incorporated in small quantities in the aqueous solution are water-miscible ethers and ketones, e.g. bu-

tanone. V

The proportions by weight of the individual components of the solutions conforming with the invention are, for example, as follows: parts by volume being cubic centimeters and parts by weight being grams:

To 100 parts by volume of water there are added approximately: up to parts by weight, preferably 1 to 4 parts by weight of boric acids and, if salts of the'boric acids are additionally used, preferably 1 to} parts by weight; up to 5 parts by weight, preferablyl to '4 parts,

by weight, of salts of boric acids, and if the salts are a used in associationwith boric acids, preferably 1 to 3 parts by weight; Sito 20, preferably-7 to 12 parts by weight of hydrophilic organic high-polymer acid. Moisture-conserving agents are in generaladded in an amount between and 50 parts by volume, preferably to 30,

, and wetting and swelling agents in quantities of up to 10 .parts by volume, tween '1 and 5 parts preferably '5 to 8 parts by volume. Beby weight of substances which form a hydrophilic film are preferably used. The quantity'of water-miscible organic solvent is not important. In general, additions of up to 10 percent by volumegive good results.

It is advantageous first to dissolve the hydrophilicorganic, high-polymer acid in water and then to add the other components to this solution.

The solution has unlimited shelf life and is easy to prepare. It is easy to handle because it does not attack the skin and does not evolve unpleasant or harmful odors or vapors. No special protective measures need be taken for its storage, such as brown-colored or other light-excluding bottles.

The invention will be further illustrated by reference to the following specific examples:

Example 1 10 parts by weight of polyvinyl phosphonic acid are dissolved in 100 parts by volume of water to which 5 parts by weight of ammonium tetraborate, 20 parts by volume of triglycol, 10 parts by volume of methyl glycol acetate and 3 parts by weight of gum arabic are added. The resultant mixture is applied to a fixed electrophotographic image by repeatedly wiping it over with a soaked cotton pad. The treated electrophotographic material can then be set up immediately and used for printing in an offset machine. The prints obtained have excellent marginal sharpness and freedom from background, and retain their quality after long printing runs.

Example 2 5 parts by weight of polyvinyl phosphonic acid are dissolved in 100 parts by volume of water to which 2 parts by weight of orthoboric sodium tetraborate, 30parts by volume of triglycol, 12 parts by volume of n-propanol and V gum arabic are added. The solution is applied to a fixed image as in Example 1. The resulting printing plate yields lOng printing runs and prints of excellent quality.

It will be'obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and a the invention includes all such modifications.

What is claimed is: p 7

1. A process for hydrophilizing the non-image areas of an electrophotographic material having a fixed image thereon which comprises treating the surface of the material with an aqueous solution including a compound selected from the group consisting of boric acid and salts thereof, a hydrophilic organic acid of high molecular weight, and at least one additive selected from the group consisting of a moisture-conserving agent, a wetting and swelling agent, and a 2. A process according to contains a water-miscible organic solvent.

3. A process according toclaim '1 in which the. boric acid is orthoboric acid. 7

4. A process according to claim boric acid is ammonium tetraborate.

5. A process according to claim 1 in which the organic acid is polyvinyl phosphonic acid.

6. A process according to claim 1 in which the additive is a mixture of triglycol, methyl glycol acetate'and gum arabic.

7. A process according to claim 1 in which the additive is a mixture of triglycol, n-propanol, and gum arabic.

8. A process according toclaim 1 in which the electrophotographic material contains a .zinc oxide photocon ductor.

9. A hydrophilizing solution for the treatment 'of elec trophotographic material comprising an aqueous solution 7 of a compound selected from the group consisting of boric 'acid and salts thereof, a hydrophilic organic. acid ofhigh'v molecular weight, and atleast one additive selected from the group consisting of a moisture-conserving agent, a wetting and swelling agent, and a hydrophilic film-forming substance. 10. A solution according addition, a water-miscible organic solvent.

11. A solution according to-claim 9 in which the boricQ acid is orthoboric acid.

12. A solution according to claim 9 in which the salt of boric acid is ammonium tetraborate. I

13. A solution according to claim 9 in which the organic acid is polyvinyl phosphonic acid.

14. A olution according toclaim 9 in tive is a mixture of triglycol, methyl glycol acetate and gum'arabic.

15. A solution according to claim 9 in which the additive is a mixture of triglycol, n-propanol, and gum arabic.

16. A printing plate prepared by the process of claim 1."

References Cited by the Examiner UNITED STATES PATENTS 7 2,988,988 6/1961 Kurz 96-1 3,017,826 1/1962 Salzberg l01'149.2

Uhlig 9633 NORMAN G. TORCHIN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner.

acid, 3 parts by weight of.

2 parts by weight of hydrophilic film-forming substance;

claim 1 in which the solution 1 in which the salt of to claim 9 containing, in

which the addi- 

1. A PROCESS FOR HYDROPHILIZING THE NON-IMAGE AREAS OF AN ELECTROPHOTOGRAPHIC MATERIAL HAVING A FIXED IMAGE THEREON WITH COMPRISES TREATING THE SURFACE OF THE MATERIAL WITH AN AQUEOUS SOLUTION INCLUDING A COMPOUND SELECTED FROM THE GROUP CONSISTING OF BORIC ACID AND SALTS THEREOF, A HYDROPHILIC ORGANIC ACID OF HIGHER MOLECULAR WEIGHT, AND AT LEAST ONE ADDITIVE SELECTED FROM THE GROUP CONSISTING OF A MOISTURE-CONSERVING AGENT, A WETTING AND SWELLING AGENT, AND A HYDROPHILIC FILM-FORMING SUBSTANCE. 